Memory devices for non-volatile storage of information are widely used. Examples of such memory devices include read only memory (ROM), programmable ROM (PROM), erasable programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), and flash EEPROM. A flash memory generally refers to a flash EEPROM, which may be erased in blocks of data instead of one byte at a time.
A flash memory device generally includes an array of memory cells arranged in rows and columns. Each memory cell includes a MOS transistor structure having a gate, a drain, a source, and a channel defined between the drain and the source. The gate corresponds to a word line, and the drain or source corresponds to a bit line of the memory array. A conventional flash memory cell generally includes a trapping layer provided between the gate and the channel. The trapping layer may be a floating gate formed of polysilicon or a dielectric such as silicon nitride. When the gate, the drain, and the source of the memory cell are appropriately biased, charge carriers (electrons or holes) may be forced to tunnel or inject into the trapping layer, which traps the carriers. As a result, the memory cell is programmed or erased. The memory cell may be read or erased by applying different biases to the gate, the drain, and the source thereof.